1. Field of the Invention
The invention resides in the field of sample introduction systems for spectrometry and more particularly relates to valves for presenting samples in aerosol or other form to inductively coupled plasma excitation devices.
2. Description of the Prior Art
Many analytical techniques for the determination of chemical species are limited in the number of samples that can be processed per unit time because of previous sample carry-over effects within the sample introduction system.
The normal procedure for presenting samples to a spectrometer excitation source, inductively-coupled plasma for example, is to first remove traces of the previous sample by pumping or aspirating a blank rinse solution through the sample introduction system. Then after a sufficient passage of time to allow for the previous sample to be effectively washed out, the system begins pumping the new sample and waits for system stabilization, known as uptake time. After the system has stabilized, the analytical data is then collected. In a simultaneous instrument the time required to collect the analytical data can be significantly shorter than the combined washout and uptake time. The present invention involves the use of two or more sample introduction units, such that while data is being collected on the sample resident in one introduction unit the other unit is washing out previous samples or is uptaking a new sample for presentation.
A number of criteria must be considered in the design of the apparatus which comprises the invention. For example, such a system or valve must not allow any undue restrictions or expansions to the aerosol flow which would permit condensation or deposition of the sample aerosol.
It must present to the aerosol the least tortuous path to the excitation source, so as to minimize bends and turns which distort the aerosol flow dynamics which could result in sample condensation or deposition.
The design must minimize the internal volume of the valve to limit as much as possible any previous sample aerosol entrapment.
The system must provide for venting the aerosol that is not traveling to the torch. It is necessary that venting occur during the valve switching so that internal pressures do not increase excessively.
The invention described below meets the above stated criteria and has the features and is constructed according to the following summary.